As a magnetic recording medium, a so-called coating-type magnetic recording medium comprising a non-magnetic support having coated thereon a magnetic layer comprising a binder and a ferromagnetic powder bonded thereto has been widely used but since the magnetic recording medium of this type uses a binder, the content of the ferromagnetic powder in the magnetic layer can not be increased beyond a certain point and hence it is difficult to increase the density of the ferromagnetic powder. Accordingly, for increasing the density of ferromagnetic powder in the magnetic layer, it has been attempted to increase the packing density of magnetic powder by introducing hydrophilic functional groups to a binder for use in a magnetic recording medium. Particularly, in the case of a vinyl chloride type resin, functional groups such as --OH and --COOH groups are introduced for increasing the packing density of the dispersed matter. However, if the vinyl chloride type resin alone is used as the binder, the performance of the medium greatly decreases in view of the running stability and the running durability because of its extremely poor flexibility. In view of the above, resins excellent in flexibility such as a polyurethane resin, a polyester resin, a phenoxy resin, an acrylonitrile rubber, etc. are used as the second ingredient for providing the magnetic recording medium with flexibility.
The above resins not containing functional groups have been used as the second ingredient. However, since still higher density has been required for magnetic recording media in recent years along with the development in short wavelength recording systems, acidic groups (such as carboxylic, sulfonic, sulfate, and phosphate groups), amino groups, imino groups, imido groups, amido groups, hydroxy groups, alkoxy groups, thiol groups, halogen groups, silyl groups, etc. have now been included in the second ingredient. Thus, the packing density of the magnetic powder can be improved so as to be suitable for the short wavelength recording system.
However, if the dispersibility is increased excessively, the surface smoothness of the magnetic recording medium is also increased, and the drawback exists that the friction coefficient is undesirably increased. Accordingly, it is difficult at present to improve the dispersibility on one hand while suppressing the increase in the friction coefficient due to the mirror-smooth surface on the other hand.
Further, it has been an intention to decrease the particle size of ferromagnetic powder used for the magnetic recording medium to permit short wavelength recording in recent years. In these circumstances, a problem such as reduction in the high frequency output and deterioration of the S/N have resulted in the problems encountered at present.
For reducing the friction coefficient, surface active lubricants, for example, fatty acids, fatty acid amides, fatty acid alcohols, fatty acid esters, or silicone oils, etc. have been used. Although the friction coefficient can be reduced to some extent by such means, it is not yet satisfactory for ensuring sufficient running performance and running durability. Further, if they are added in an excess amount, disadvantages such as head contamination occur due to exudation of the lubricants, and this can not be considered an effective means.
In increasing the density of magnetic recording media in recent years, their friction coefficient tends to be increased by making the magnetic particles finer, increasing the packing density, and providing a mirrorsmooth surface. However, both satisfactory running . stability and satisfactory electromagnetic conversion properties are difficult to attain by the mere application of the surface active agents, carbon black, or inorganic powders. Further, there has been a great problem in recent years that the high frequency output and the S/N of the magnetic medium running in a deck are deteriorated.
In order to avoid these problems, methods have been proposed in which polar groups, such as a sulfonate group, a sulfate group, a carboxylate group, a phosphate group, etc., are introduced into the binder, but they are still insufficient in view of the electromagnetic conversion properties (as described in U.S. Pat. Nos. 4,529,661, 4,521,486, and 4,613,545).
The present inventors have made earnest studies on such problems and, as a result, have found that if the frictional resistance between the surface of the magnetic recording medium and the glass portion at the periphery of a magnetic head is high, destruction is caused at the surface of the glass portion by the high speed sliding contact of the medium and, as result, fine protrusions are formed on the glass surface and roughen the glass surface. It has been found that the fine protrusions come together with the running time, which increases the spacing loss between the magnetic surface of the medium and the magnetic head to result in the reduction in the high frequency output and the S/N. It has also been found that this phenomenon is greatly influenced particularly by the grain size of the ferromagnetic powder and remarkably occurs in fine magnetic particles having a specific surface area of 40 m.sup.2 /g or more as measured by the BET method.